Polymers: Addition vs Condensation

Polymers are large molecules made of many repeating units called monomers. The 0620 syllabus covers two types of polymerisation, and the exam frequently tests the ability to identify the monomer from the polymer (and vice versa), draw repeating units, and compare the two mechanisms.

Addition polymerisation

How it works

Addition polymerisation uses alkene monomers (molecules with a C=C double bond). The double bond opens up, and many monomers join together in a long chain. No other product is formed.

n(CH2=CH2) -> -(CH2-CH2)-n

The monomer is ethene (C2H4). The polymer is poly(ethene), commonly called polythene.

Key features of addition polymerisation

• Monomer must contain a C=C double bond (unsaturated)
• The double bond opens to form single bonds linking monomers
• Only one product (the polymer) — no small molecule lost
• The repeating unit has the same atoms as the monomer, just rearranged from double to single bonds

Common addition polymers

Monomer	Polymer	Uses
Ethene (CH2=CH2)	Poly(ethene)	Plastic bags, bottles, packaging
Propene (CH3CH=CH2)	Poly(propene)	Rope, carpets, containers
Chloroethene (CH2=CHCl)	Poly(chloroethene) / PVC	Pipes, window frames, insulation
Tetrafluoroethene (CF2=CF2)	PTFE / Teflon	Non-stick coatings

Drawing the repeating unit from the monomer

1. Start with the monomer
2. Open the double bond to make two single bonds
3. Show the bonds extending out of both sides of the unit with continuation bonds
4. Put brackets around the unit with subscript n

Identifying the monomer from the polymer

1. Look at the repeating unit inside the brackets
2. Find the backbone C-C single bond that was originally the double bond
3. Replace it with C=C
4. That is the monomer

Condensation polymerisation (Supplement)

How it works

Condensation polymerisation joins monomers together with the loss of a small molecule (usually water, H2O). Each monomer has two functional groups so the chain can grow from both ends.

Types of condensation polymer

Polyesters: Formed from a diol (two -OH groups) and a dicarboxylic acid (two -COOH groups). Water is lost at each ester linkage.

HO-R-OH + HOOC-R’-COOH -> -O-R-OOC-R’-CO- + H2O

The ester link is -COO-.

Polyamides (e.g. nylon): Formed from a diamine (two -NH2 groups) and a dicarboxylic acid (two -COOH groups). Water is lost at each amide linkage.

H2N-R-NH2 + HOOC-R’-COOH -> -NH-R-NHCO-R’-CO- + H2O

The amide link is -CONH- (also written -NHCO-).

Proteins are natural polyamides made from amino acid monomers. Each amino acid has both an -NH2 and a -COOH group.

Key features of condensation polymerisation

• Monomers have two functional groups each
• A small molecule (usually water) is released at each link
• The repeating unit has fewer atoms than the monomers combined (the water atoms are removed)
• The linkage can be identified: ester (-COO-) or amide (-CONH-)

Comparison table

Feature	Addition	Condensation
Monomer type	Alkene (one type with C=C)	Two types with functional groups (or one type with two different groups)
Small molecule produced?	No	Yes (usually water)
Linkage in polymer	C-C single bonds	Ester (-COO-) or amide (-CONH-)
Example polymer	Poly(ethene)	Nylon, polyester, proteins
Saturated or unsaturated monomer?	Unsaturated	Saturated

Environmental impact of polymers

Problems with addition polymers

• Non-biodegradable: Microorganisms cannot break down the strong C-C backbone
• Persist in landfill: Take hundreds of years to decompose
• Toxic fumes if burned: Incomplete combustion produces CO; PVC releases HCl
• Litter: Pollutes oceans and harms wildlife

Solutions

Approach	Description
Recycling	Melt and remould (mechanical recycling). Sorts by polymer type
Incineration	Burn for energy. Must scrub toxic gases
Biodegradable polymers	Made from plant-based materials; broken down by microorganisms
Reduce use	Use alternatives (paper bags, glass bottles)

Condensation polymers can be broken down by hydrolysis (reversing the condensation reaction with water), making them potentially more environmentally friendly, though many are still persistent.

Natural polymers

Polymer	Monomer	Type
Starch / cellulose	Glucose	Natural condensation (polysaccharide)
Proteins	Amino acids	Natural condensation (polyamide)
DNA	Nucleotides	Natural condensation
Rubber	Isoprene	Natural addition

Common exam mistakes

1. Saying addition polymerisation produces water — no. Only condensation polymerisation produces a small molecule. Addition has only one product.
2. Forgetting to open the double bond when drawing the repeating unit — the C=C becomes C-C in the polymer.
3. Not knowing which monomer type goes with which polymerisation — alkenes = addition; molecules with two functional groups = condensation.
4. Confusing biodegradable with recyclable — biodegradable means broken down by microorganisms. Recyclable means can be reprocessed.

Worked exam questions

Draw the repeating unit of the polymer formed from propene (CH3CH=CH2). Name the polymer. State the type of polymerisation. [3 marks]

• Repeating unit: -(CH(CH3)-CH2)- with continuation bonds on each side [1]
• Name: poly(propene) [1]
• Type: addition polymerisation [1]

Explain why a polyester is a condensation polymer rather than an addition polymer. [2 marks]

• A small molecule (water) is produced when the monomers join [1]
• The monomers are not alkenes / do not contain a C=C double bond; they have two functional groups (-OH and -COOH) [1]

State two environmental problems caused by disposing of addition polymers. Suggest one way to reduce these problems. [3 marks]

• Addition polymers are non-biodegradable / persist in the environment for a long time [1]
• Burning them produces toxic/harmful gases (e.g. CO, HCl from PVC) [1]
• Recycle them / use biodegradable alternatives / incinerate with gas scrubbing [1]